To date, biometric authentication technology using biometric information of individuals, such as fingerprints, faces, or veins, has been utilized as a way of verifying the identity of a person without anybody's help when that person enters and exits a facility. Unlike a magnetic card or a personal identification number (PIN), biometric authentication using biometric information has the advantage that there are no concerns about loss (forgetfulness) or theft. In the biometric authentication technology, authentication is performed by performing comparing between biometric information registered in advance (registered data) and biometric information presented at the time of authentication (comparing data). However, biometric information varies to some extent even if the information is on the same person, and therefore some variations have to be permitted for authentication. Typically, the similarity between both is expressed in terms of the degree of similarity, and a person who presented biometric information is identified as a registered person him- or herself if the degree of similarity is greater than a given threshold. In biometric authentication, it is desired that accuracy in authentication be high. For this reason, methods for raising the degree of similarity between registered data and comparing data for the same person are used. The methods for raising the degree of similarity between registered data and comparing data are broadly classified into those performed during feature extraction and those performed during a comparing process.
Normalization is an example of a method performed during feature extraction. This is a technique of combining images or biometric information obtained when a posture (position, orientation, shape) of a human body presented to a photographic device is captured in such a manner that the posture is changed to a normal posture. Japanese Examined Patent Application Publication No. 2-61790 discloses a method in which the outline of a target is detected and, based on this, the amount of position displacement of the target is determined. Japanese Laid-open Patent Publication No. 2002-92616 discloses a technology in which the outline of a finger is detected and, based on this, errors in the location, as well as the orientation, of a vein image are corrected. Japanese National Publication of International Patent Application No. 5-508951 discloses a scheme in which locations of parts, such as eyes, arranged in a face are determined and the locations are normalized. Japanese National Publication of International Patent Application No. 2000-511316 discloses a scheme in which the relationship between an amount that may be computed from an image and hidden data representing a posture is learned using training data.
In contrast, alignment based on template comparing is an example of a method performed during a comparing process. Template comparing is used in the image-processing field in general (for example, refer to Japanese Laid-open Patent Publication No. 7-129770). For example, Japanese Laid-open Patent Publication No. 7-129770 discloses a method for removing the influence of misalignment between registered data and comparing data by repeatedly calculating the degree of similarity between the registered data and the comparing data while gradually changing the positional relationship between the registered data and the comparing data, and regarding the maximum value of the calculated degrees of similarity between the registered data and the comparing data as the final degree of similarity between them.